A thermally conductive sheet is used in such a condition that it is arranged to be in contact with a heat-generating body such as electronics. Therefore, a thermally conductive sheet is required to have superior thermal conductivity and high flame retardancy. The flame retardancy is desired to correspond to “V-0” level in UL Flameproof Test Standard UL-94 (Underwriters Laboratories, Inc. Standard No. 94) “flammability test of plastic materials for devices and electronic parts (hereinbelow referred to as “UL-94 flammability test”).”
On the other hand, with recent miniaturization and high integration of electronics, requirement for a thermally conductive thin sheet having a thickness of below 1 mm has been increasing. As a binder constituting a thermally conductive sheet, a silicone resin has recently been used. However, there is an indication that siloxane gas generated from the silicone resin causes a contact failure in electronics. Therefore, it is preferred to use a non-silicone resin.
However, it has been difficult for a conventionally-known thermally conductive sheet containing a non-silicone resin as a binder to have a flame retardancy corresponding to V-0 in UL-94 flammability test when the sheet has a thickness of less than 1 mm. In addition, when a thermally conductive sheet is made so thin, the thermally conductive sheet has relatively low strength. Therefore, it has been difficult to simultaneously achieve superior adhesion with electronics, or the like, and handleability of the thermally conductive sheet. Incidentally, it is possible to blend a halogen-type flame retardant (retardant containing halogen) or a red phosphorous with the binder to enhance flame retardancy while the thermally conductive sheet is kept thin (refer to JP-A-2003-238760). However, there is a request of avoiding such a halogen-containing flame retardant by the industrial world in consideration of influence on the environment. In addition, use of a red phosphorous is not always preferable in view of safety.
Related prior arts are disclosed in, for example, JP-B-3283454 and JP-A-63-118392. In addition, a radiating sheet which comprises an elastic substrate and an adhesive layer, each containing a filler having high thermal conductivity is disclosed in, for example, JP-A-11-74667. However, since the adhesive layer constituting the radiation sheet is unvulcanized, it is prone to cause a flame drip in a perpendicular flaming test such as UL-94 flammability test. Therefore, it has been difficult to achieve flame retardancy corresponding to V-0 in UL-94 flammability test when the radiating sheet is made to have a thickness of below 1 mm. Further, the unvulcanized adhesive layer sometimes causes adhesive transfer when the sheet is peeled off from the object since the adhesive layer is unvulcanized. In addition, the radiation sheet disclosed in JP-A-11-74667 has insufficient thermal conductivity because of a low coefficient of thermal conductivity thereof.
In addition, an adhesive tape which comprises an elastic substrate and an adhesive layer, each containing a filler having thermal conductivity and electrical insulation, is disclosed in, for example, JP-B-2003-160775. However, the adhesive tape has a problem of not showing sufficient flame retardancy in UL-94 flammability test. In addition, the adhesive tape disclosed in the document has insufficient thermal conductivity because of a low coefficient of thermal conductivity thereof.